A video image monitoring system has functions of conducting an image processing on a video image obtained by an image-capturing device such as a camera etc.; and detecting a mobile object such as a person or a vehicle appearing in a monitored area. The video image monitoring system of this kind has various functions of recording only a video image in which a mobile object appears therein; displaying a warning icon on a display device; and giving an alarm to a security personnel by sounding an alarm etc. The video image monitoring system of this kind also lowers the workload of security personnel in a monitoring operation, which formerly required continuous observation. In addition, the video image recorded by using the video image monitoring system of this kind can be used for investigating a criminal act such as theft, or an illegitimate act.
Recently, more and more image monitoring systems are introduced in mass merchandisers, financial institutions, buildings, and offices, since awareness of crime prevention increases in society because of crime rate growth, expansion of crime patterns, and lowering criminal arrest rate. More and more cameras are installed at various locations since the storage capacity of a video recording device increases and since network cameras are widely used. There is a growing demand for a surveillance assist function since it is too burdensome for a security personnel to monitor a recorded video image continuously to find out a criminal act etc. as previously explained.
In addition, the video image monitoring system has expanded its scope of application into combined use with a pan, tilt, zoom (PTZ) camera, having a zoom lens unit and mounted on a camera platform capable of being rotated and tilted, for tracking a trespasser, or with a camera mounted on a mobile robot etc. for the purpose of visual recognition. However, these applications have a problem since not only an object to be monitored but also a background moves in the field view of a camera if the camera is moved. In a known image recognition processing method which is ordinarily employed for a fixed camera for detecting a mobile object, a reference background image is produced at first, and then, a difference between the reference background image and an image input anew is calculated.
A conventionally known image-processing device (see Patent Document 1) detects changes among images produced from a video image captured by the PTZ camera, and then, estimates a camera framing based on the result of the detection. The previously produced images are transformed based on the estimated camera framing, and then, images of a mobile object is extracted by using the transformed images and the video image captured by the camera. The image-processing device conducts an image recognition process to the extracted images of the mobile object.
In addition, an image-processing operation must be stable against a varying brightness or noise when the image-capturing condition varies, e.g. in an outdoor. A known method disclosed in Non-Patent Document 1 is robust against the varying brightness since brightness gradients in the images are encoded to direction code data in this method.